As a solid state drive (SSD) using a NAND type flash memory as a storage medium becomes widespread, the use of the SSD has become common instead of a hard disk drive (HDD) as the storage medium in a storage system. Compared with the HDD, the SSD can implement a fast access and can be installed as the memory device of the storage apparatus, such that the speed of the storage apparatus can be increased. In addition, development for practical use of the SSD including a nonvolatile semiconductor memory capable of faster access than flash memories, such as a resistance random access memory (ReRAM) and a phase change random access memory (PRAM) is under development.
On the other hand, the SSD has higher price per capacity than the HDD. Therefore, in the case of using the SSD, technologies of reducing a capacity of stored data such as data compression and de-duplication are important. For example, PTL 1 discloses a technology of providing a larger logical capacity than a physical capacity of a semiconductor storage medium included in the SSD by compressing data stored in the SSD and reducing the capacity of the used semiconductor storage medium.
In addition, there is a certain limitation on a rewriting frequency of the nonvolatile semiconductor memory which is the storage medium of the SSD. That is, the SSD has a life span. For example, if the writing frequency for the nonvolatile semiconductor memory exceeds the predetermined number of upper limit, it is not possible to normally read and write data from and into the nonvolatile semiconductor memory. In addition, the variation in quality of the nonvolatile semiconductor memory is becoming larger with the progress of fine processing. Therefore, some of the nonvolatile semiconductor memories in the SSD may not be used.
For example, PTL 2 discloses a technology for reducing a logical capacity provided by the SSD and maintaining a function when some of the nonvolatile semiconductor memories cannot be used due to a failure.